The present invention generally relates to printed circuit connectors, and more specifically to a prefabricated strip of printed circuit fingers or contacts and the method of mounting the same on a printed circuit board.
Electronic circuits, and more particularly the more complex circuits, frequently employ one or more printed circuit boards on which various components or elements are mounted. In order to interconnect the various printed circuit boards, as well as for feeding electrical power and signals to the various circuit elements, and for extracting signals therefrom, various connector arrangements are utilized. Since printed circuits are normally formed on boards or laminate sheets, made of various epoxy compositions or fiberglass and relatively thin layers of copper which have been etched or deposited to define the desired circuit, the problem in each case is one of coupling from the relatively thin circuit conductor leads which are "printed" on the board and a mechanical connector which is generally three-dimensional.
The most frequently used printed circuit connectors being manufactured today fall into three broad design categories: (1) the "edge" type in which the edge of the circuit board mates with a slotted connector; (a) a "two-piece" type where one mating piece is permanently attached to the circuit board; and (3) a "hermaphroditic" connector which combines separately mounted contact banks and a card slot. The present invention is concerned with all three types. The examples illustrated primarily show the edge type in which the edge of the circuit board mates with a slotted connector. The slotted connectors may include various contact configurations, to be more fully described in the specification.
A problem has existed and continues to exist with respect to the formation of the printed circuit fingers, contacts or terminals which mate with the card-edge connectors. As will be more fully described in connection with FIGS. 17-20, the known methods of forming the fingers on cards, including plating the fingers and otherwise imparting desirable characteristics thereto, is a complex procedure requiring many and costly steps. Additionally, the commonly used technique for forming such printed circuit fingers on circuit board suffers the disadvantage that the "printed" circuit on the board is frequently damaged during the formation or processing of the fingers so that numerous printed circuit boards must be rejected because they fail the quality control criteria for such boards. The high rate of rejection of the boards results, of course, in a substantial increase in the cost of manufacture of such printed circuit boards.
Due to the cumbersome and expensive method of forming printed circuit fingers suitable for use with edge connectors, various devices and methods have been proposed to facilitate the procedure as well as to reduce the high rejection rate. However, numerous problems have persisted and the cost and rejection rate are still unsatisfactory with the presently known and commonly used techniques.
Due to the complex and expensive procedure above suggested in the formation of contact terminals or fingers for printed circuit boards, the techniques and procedures used in the manufacture of the commercial or finalized boards could not practically be used to any great extent during the bread-boarding or development stages. There have been proposed various printed circuit aids, of the dry transfer of pressure-sensitive type, which are provided in bread-boarding kits for printed circuit boards. Examples of such bread-boarding aids are disclosed in U.S. Pat. Nos. 3,538,389 and 3,703,603. The earlier patent is entitled "Sub-element for Electronic Circuit Boards" and discloses sub-elements in the form of thin epoxy or plastic strips, having a basic circuit formed thereon by printed circuit techniques. To assist in the mounting of the sub-elements, the thin strips are backed with a pressure-sensitive or other type of adhesive which forms a backing for the strips. It is recommended in that disclosure that the adhesive backing be covered with a release or backing paper or sheet. However, with the sub-elements disclosed in the earlier patent, the carrier or strip which carries the sub-elements or conductors remains attached to and becomes part of the printed circuit board configuration after the conductor elements are mounted on the circuit board. While this may be satisfactory for bread-boarding purposes, it is not satisfactory for final assembly where the conductive elements must be permanently and reliably fixed to the printed circuit board.
The later patent discloses the use of sub-elements which may be adhesively attached to a panel-like base of circuit board. The sub-elements are in the form of conductive metallic particles which are adhesively attached to a carrier strip, such as plastic or wax paper, the particles being intermixed with adhesive material. The sub-elements are formed of a basic compound which include, for example, copper, adhesive, adhesive activating and electrolyte or other conductor particles. The basic compound is screened on to a carrier with a releasing agent coating being provided between the carrier and the sub-elements formed out of the compound. A release strip, in the form of wax paper, plastic, parchment or tissue is provided on the other side of the sub-elements so that the sub-elements are in effect sandwiched between a release strip and a carrier strip. The sub-elements are applied to a circuit board laminate sheet by first removing the release strip. The basic compound design is then placed on the surface of the laminate sheet and the top surface of the carrier is rubbed by a suitable tool. The resulting heat and pressure causes the compound design to become adhesively attached to the laminate sheet, with the heat generated by the rubbing action melting the release agent coating which exists between the compound design and the carrier. The carrier can then be removed from the resulting circuit.
While the rub-on sub-elements disclosed in the above patents facilitate bread-boarding of printed circuits, the sub-elements and methods of mounting the same disclosed in the above patents are not suitable for the manufacture of final assemblies where reliability is of primary concern. Additionally, the rub-on sub-elements require various steps to mount the sub-elements on the printed circuit boards, some of which steps must be performed manually and therefore the known printed circuit sub-elements cannot and are not intended to be automatically mounted during mass production. Being intended primarily for bread-boarding, the somewhat complicated construction of the rub-on sub-elements as well as the high cost thereof is tolerable. However, such rub-on sub-elements become impractical and overly expensive for mass production runs. The sub-elements of the type disclosed in the above patents are typically provided on carriers which have limited dimensions and this limits the number of connector sub-elements which can be mounted on a circuit board for use with edge-type printed circuit connectors. A primary disadvantage, however, of the sub-elements disclosed in the above patents, is that they cannot be mounted on circuit boards with the degree of permanency which is required to provide a reliable finished assembly. Another advantage over the prior art is that the invention lends itself to semi-automatic or automatic application of the contacts, and permits extremely accurate alignments of the contacts, regardless of however small the center to center spacing is and/or how narrow the width of the contacts are.